1. Field of the Invention
The present invention relates to a display device which can be used for audio visual equipment, office automation equipment or computers.
2. Description of the Related Art
In recent days, as the society keeps more highly informationized, demands are going up for a large-size and large-capacity display. In response to such demands, a CRT (cathode-ray tube) currently referred to as "King of a display" has developed to become more highly definitive and larger in size. The direct-view display reaches a 40-inch display and the projection display reaches a 200-inch display. However, the weight and the depth of the CRT itself becomes serious in light of realizing the large-capacity display. To overcome these serious respects, the drastic solution has been strongly required.
Steady study on a plane type display arranged to display an image on a different principle from the conventional CRT has been progressed to keep the display quality so high that the plane type display can be escaped from the current narrow range of uses for a wordprocessor machine and a personal computer and can be used for a hi-vision or a high-efficiency engineering workstation display.
As a plane type display, it is possible to refer to an ELP (electroluminescent panel), a PDP (plasma display panel), a VFD (fluorescent display), an ECD (electrochromic display) or a LCD (liquid crystal display). Of them, the LCD is the most promising device from viewpoints of feasibility of a full-color display and a matching characteristic to an LSI (Large Scale Integration). Under the circumstance, the technical progress for the LCD is the most remarkable.
The LCD is divided into a simple matrix driving type LCD and an active matrix driving LCD. The simple matrix driving LCD is structured to have an XY matrix panel composed of a pair of glass substrates having striped electrodes formed on each of them and a liquid crystal material injected therein. The acuteness entailed in the liquid crystal display characteristic is used for actuating the display function.
On the other hand, the active matrix driving type LCD is structured to directly add non-linear elements to pixels and use the non-linear characteristic of each element (switching characteristics or the like) for actuating the display function. As compared to the former simple matrix driving type LCD, therefore, the dependency of this LCD on the display characteristic of the liquid crystal itself is smaller, so that this LCD can realize a high-contrast and a highly responsive display. This kind of a non-linear element is divided into two-terminal type and a three-terminal type. As a two-terminal type non-linear element, an MIM (metal-insulation-metal) or a diode has been proposed. As a three-terminal type non-linear element, a TFT (thin-film transistor), an Si-MOS (silicon metal oxide semiconductor), or an SOS (silicon-on-saffire) has been proposed.
However, the simple matrix driving type LCD and the active matrix driving type LCD both involve such disadvantages in light of contrast, response speed and reliability as disabling to realize a large-size and large-capacitance display. This results in being unable to realize a sufficient performance of such a display.
That is, for the simple matrix driving type LCD, a ratio of an effective voltage applied into a selective pixel electrode to that applied into a non-selective pixel electrode comes closer to 1 as the number of scan lines is increased. This needs the liquid crystal itself to have an acute display characteristic. However, the feasible acuteness is limited. In the current state, the number of the scan lines is limited to about 400. Further, the response speed is likely to be contrary to the acuteness of the display characteristic. It means that the response speed is slower as the scan lines are increased in number. In general, when 400 scan lines are provided, the response speed is as slow as 100 ms to 300 ms. To realize the large-capacitance display, a method is often used where the signal lines are divided into an upper part and a lower part on a panel display and both of the upper and the lower parts are independently scanned. Concretely, if 400 scan lines are provided when not divided, the apparent number of the scan lines becomes 800. At a trial stage, the display capacitance consisting of 800.times.1024 lines is realized.
However, this method needs two drivers for data signal lines for the upper and the lower blocks. Hence, the number of the drivers is twice as many as that of the normal method, resulting in making the display more costly. Further, the display employing this method is superior in contrast and response speed to the active matrix driving type LCD. As the panel is made larger in size, the wiring length of the transparent electrode is made larger. This results in increasing the wiring resistance, thereby lowering the unevenness and the contrast on the display resulting from the attenuation of the data signal. Hence, for realizing the large-size and large-capacitance display, a drastic breakthrough is necessary.
On the other hand, for the active matrix driving type LCD, the switching characteristic and the non-linear characteristic of the non-linear element have been actively used. It is less remarkable in the quality of display than the simple matrix driving type LCD when the display has a large capacitance. However, in actual, the parasitic capacitance exists in the non-linear elements through the scan lines. Hence, the leakage of a scan electric signal to the pixel electrode lowers the contrast, brings about an after image, and lowers a lift of a panel. When increasing the display in size, the wiring length is made longer so that the increase of the wiring resistance and the parasitic capacitance cause a signal to be attenuated on the signal line. This has a great adverse effect on the evenness and the contrast of the display. Hence, for this type of LCD, an epochal new technology is promising for realizing the large-size and large-capacitance display.
Incidentally, viewing the market and use of the LCDs, a lap-top computer and a palm-top computer having the simple matrix driving type LCD and the active matrix driving type LCD mounted thereon are promising goods for attracting the market of the LCDs. In those goods, in particular, a portable data terminal such as a palm-top computer, the data input unit is important. To save the keyboard space, a handwriting input function is strongly desired.
As this function, it has been conventionally proposed to locate a tablet digitizer outside of a liquid crystal display and sense a location where data is input by a pen through the effect of the tablet. This tablet is an electromagnetic induction type, a resistance pressure-sensitive type and an electrostatic connecting type.
The electromagnetic induction type enables to realize a relatively high resolution but, as disadvantages, consumes a large power and is heavy. Further, it is easily influenced by a magnetic field.
The resistance pressure-sensitive type is so simple in structure that it is more advantageous in light of power consumption and weight. But, as disadvantages, it provides a low resolution and durability.
The electrostatic connecting type enables to realize a relatively high resolution but, as disadvantages, has difficulty in realizing a wireless pen and consumes a relatively high consumption. Further, it is weak to static electricity.
That is, currently, none of these types meet the needs and apply to a large-size liquid crystal panel.